What is the ruling on Dhihar, and what is its expiation if one was coerced into swearing by it?

If a person is coerced into performing ẓihār under duress, his ẓihār is not valid according to many scholars. However, it is valid in the Hanafi madhhab, as they do not stipulate that ẓihār must be voluntary or intentional. Thus, the ẓihār of one who is coerced or mistaken is valid, unlike the Shafi'is who do stipulate that.